1. Field of the Invention
The present invention relates to an optical scanning apparatus that exposes an object to be exposed using light beams.
2. Description of the Related Art
Conventionally, as methods to expose a photosensitive drum in an optical scanning apparatus in an image forming apparatus of an electrophotographic type or the like, laser beams emitted from a semiconductor laser are often used. Most of laser elements of an edge emitting type used in this field are configured to output front light emitted from a front surface of the element and rear light emitted from a rear surface of the element. The rear light is received in a photodetector (PD) that functions as a light-sensitive element, and variation of an amount of the laser beam emitted from the semiconductor laser is monitored. The amount of the laser beam is controlled to be constant by an auto power control (APC) that performs light amount correction. The APC is performed since the laser elements generally have a temperature characteristic that as temperature of the elements increases, the amount of the laser beams to be output decreases.
Meanwhile, in apparatuses such as compact discs (CDs) and digital versatile discs (DVDs), or some printers, emission of rear light from laser elements is eliminated and light of 100% laser amount is emitted from surfaces of the elements as front light. In such printers, a half mirror is disposed between the laser element and an exposure surface of a photosensitive drum, to introduce a part of light to be emitted on the exposure surface to a PD in order to monitor the amount of laser beams.
In order to effectively use the laser beam for image formation, it is necessary to monitor the light amount using a configuration which can minimize a loss on an optical path from the laser element to the exposure surface on the photosensitive drum. However, in the above-described laser emission light amount monitoring methods using the rear light or the half mirror, apart of the amount of the laser beam emitted to the exposure surface is used for monitoring. Accordingly, the amount of the laser beam emitted onto the exposure surface is reduced. Further, in the laser emission light amount monitoring method using the rear light, a correlation between the amount of laser beam emitted onto the exposure surface and the rear light may vary.
Japanese Patent Application Laid-Open No. 6-164070 discusses a use of invalidated vignetted light other than light beams used for exposure as a technique which can perform light amount correction without reducing light use efficiency.
FIGS. 12A and 12B illustrate essential configurations of an optical scanning apparatus applied to conventional image formation discussed in Japanese Patent Application Laid-Open No. 6-164070. FIG. 12A is an overall perspective view of the optical scanning apparatus. FIG. 12B is a cross-sectional view illustrating a characteristic part.
As illustrated in FIG. 12A, a divergent laser beam emitted from a laser element 601 is converted into a parallel light beam by a collimator lens 602. The converted parallel light beam is shaped by passing through an opening of an aperture 603 and enters into a cylinder lens 604. The light beam exited from the cylinder lens 604 arrives on a deflection and reflection plane of a polygonal mirror 605 that is rotationally driven, and is deflected for scanning. The deflected light beams 606 form an image on a photosensitive drum surface via an image forming optical system, and image information is recorded.
As illustrated in FIG. 12B, the aperture 603 is tilted at a 45 degree angle and supported. An incidence plane side is formed as a mirror plane, and configured to be able to reflect a vignetted light 610 that did not pass through the opening in a perpendicular direction as a detection light beam. According to this configuration, the vignetted light 610 is reflected on the mirror plane of the aperture 603 as the detection light beam, condensed by a condenser lens 611, and enters into a PD 612. Then, using a detection output of the PD 612, the above-described APC is performed.
However, the technique discussed in Japanese Patent Application Laid-Open No. 6-164070 involves the following problems.
That is, it is necessary to apply a mirror finish on the surface of the aperture 603. Further, it is necessary to condense the vignetted light 610 reflected on the surface of the aperture 603 using the condenser lens 611 and introduce the condensed light to the PD 612. Accordingly, as compared to conventional apparatuses, necessary parts and the costs will increase. Further, it is necessary to precisely position the PD 612 to introduce the condensed light to the PD 612.